1. Field of the Invention
The present invention relates to a voltage measuring apparatus and a power conversion apparatus for an electric vehicle that uses the voltage measuring apparatus.
2. Description of Related Art
Electronic equipment that includes elements generating a large amount of heat is often subjected to temperature measurement of the elements. When the temperature exceeds a specified threshold, some kind of process such as generation of an alarm or limitations on electric power flowing to the electronic equipment is frequently executed in the electronic equipment. Hereinafter, the detection in which the temperature of the element exceeds the specified threshold is referred to as a “temperature abnormality detection”.
One of the devices that generate a large amount of heat is a power conversion apparatus that is equipped in the electric vehicle. The power conversion apparatus in the electric vehicle converts electric power of a battery into alternating-current (AC) power for driving a drive motor. Such an element in the power conversion apparatus handles a large amount of electric power and thus generates a large amount of heat. In particular, switching elements in a voltage converter or an inverter and a capacitor for smoothing output current of the battery or the voltage converter generates a large amount of heat, and their heat resistant temperatures are low. Then, when the temperature of the capacitor exceeds the specified threshold (that is, the temperature abnormality of the capacitor is detected), some kind of process is executed.
A thermistor is often used for the temperature abnormality detection of the capacitor. The thermistor is a resistor in which the resistance sharply changes when its temperature exceeds a predetermined temperature. That is to say, the temperature abnormality in adjacent areas of the thermistor can be detected, based on the resistance of the thermistor. Techniques for the temperature abnormality detection by means of the thermistor are disclosed in Japanese Patent Application Publication No. 2009-111370 (JP 2009-111370 A) and Japanese Utility Model Application Publication No. 03-4274 (JP 03-4274 U), for example.